PART I. CHAPTER VIII. 



Ill 



Columnar Structure of Volcanic Rocks. 



but they have most commonly from five to seven sides. They 

 are often divided transversely, at nearly equal distances, like the 

 joints in a vertebral column, as in the Giant's Causeway, in Ire- 

 land. They vary exceedingly in respect to length and diameter. 

 Dr. MacCulloch mentions some in Sky which are about 400 feet 

 long ; others, in Morven, not exceeding an inch. In regard to 

 diameter, those of Ailsa measure nine feet, and those of Morven 

 an inch or less.* They are usually straight, but sometimes 

 curved ; and examples of both these occur in the island of StafFa. 

 In a horizontal bed or sheet of trap the columns are vertical ; in 

 a vertical dike they are horizontal. Among other examples of 

 the last-mentioned phenomenon is the mass of basalt, called the 

 Chimney, in St. Helena (See Fig. 

 98.), a pile of hexagonal prisms, 64 

 feet high, evidently the remainder 

 of a narrow dike, the walls of rock 

 which the dike originally traversed 

 having been removed down to the 

 level of the sea. In Fig. 99. a small 

 portion of this dike is represented on 

 a less reduced scale, f 



It being assumed that columnar 

 trap has consolidated from a fluid 

 state, the prisms are said to be al- 

 ways at right angles to the cooling 

 surfaces. If these surfaces, there- 



Fig. 98. 



Volcanic dike composed of horizontal fore, instead of being either perpen- 

 prisms. St. Helena. dicular or horizontal, are curved, tho 



Fig. 99. 



columns ought to be inclined at every angle 

 to the horizon ; and there is a beautiful ex- 

 emplification of this phenomenon in one of 

 the valleys of the Vivarais, a mountainous 

 district in the South of France, where, in the 

 midst of a region of gneiss, a geologist en- 

 counters unexpectedly several volcanic cones 

 of loose sand and scorise. From the crater 

 of one of these cones, called La Coupe 

 d'Ayzac, a stream of lava descends and occupies the bottom of 

 a narrow valley, except at those points where the river Volant, 

 or the torrents which join it, have cut away portions of the solid 

 lava. The accompanying sketch (Fig. 100.) represents the rem- 

 nant of the lava at one of the points where a lateral torrent joins 



Small portion of the dike 

 Fig. 98. 



* MacCulloch, Syst. of Geol. vol. ii. p. 137. 

 t Scale's Geognosy of St. Helena, plate 9. 



